Buck power factor correction system

ABSTRACT

The invention disclosed buck power factor correction system. The system includes: a first storing device, for storing and discharging energy; a first converter device, coupled to the first storing device, for transferring and converting energy; a second storing device, coupled to the first storing device, for storing and discharging energy; and a second converter device, coupled to the second storing device, for transferring and converting energy.

TECHNICAL FIELD

The present invention relates to a buck power factor correction system, and more particularly to a buck power factor correction system capable of reducing reactive power of an electronic system for providing an effect of power factor correction.

TECHNICAL BACKGROUND

A great number of current electrical appliances operate on direct current, and thus need direct-alternating current conversion since public electricity is alternating current. To reduce reactive power of an electronic system as well as to minimize current harmonics that cause system interference, a power factor corrector is prevailingly implemented in many electrical appliances that are required to have a high power factor and low current harmonics. A common power factor correction circuit stereotypically adopts a boost approach, which is however set back by a limitation that a direct-current output voltage is necessarily higher than a peak value of an alternating-current input voltage. Further, although other circuits capable of outputting a lower voltage by means of buck or buck-boost are available, these circuits suffer from drawbacks from having less satisfactory characteristics and efficiency, a large volume for a corresponding storage component, complex control means to low feasibilities.

FIG. 1A shows a boost converter circuit frequently adopted by a conventional power factor corrector, which is advantaged by having a higher power factor and simpler control means. FIG. 1B shows a schematic diagram of waveforms of an input voltage V_(s) and a current I_(s) of the conventional power factor corrector in FIG. 1A, where ω is an angular frequency of public electricity, and V_(m) and I_(m) respectively represent a voltage peak and a current peak. A current path of the boost converter circuit allows partial energy from a power source AC to directly charge a direct-current link capacitor C_(DC) (as shown by a solid-line arrow in FIG. 1A). Therefore, an energy-storing inductor in the boost converter circuit only needs to store relatively lower energy, so that it has not only a smaller volume but also high efficiency. Q_(PFC) in FIG. 1A represents an active switch transistor, D_(PFC) represents a diode, and a capacitor C_(s) may be designed based on actual requirements. However, accompanied with a high output voltage, power components of the above conventional power factor corrector are often encountered with a higher voltage stress. In addition, for a load with a lower voltage requirement (lower than a peak voltage of the power source), the conventional boost power factor corrector, instead of directly providing an appropriate power source, is only able to provide a rated voltage needed by the load after stepping down its output voltage via a buck converter circuit, as shown in FIG. 2. Yet, the above design increases a circuit size and production costs as well as circuit power consumption, such that conversion efficiency of an overall circuit is reduced as a result.

To optimize conversion efficiency of a circuit, a power factor corrector with a design of a buck converter circuit has also been proposed, as shown in FIG. 3A. A current path of a buck converter circuit also allows partial energy from the power source AC to directly charge the direct-current link capacitor C_(DC). Therefore, the energy-storing inductor in the buck converter circuit only needs to store relatively lower energy, so that it has not only a smaller volume but also high efficiency. A main shortcoming of the buck power factor corrector is that, when a power source voltage is lower than a direct-current output voltage, the circuit fails to induce an input current such that the input current becomes discontinuous, as shown in FIG. 3B. As a result, the buck power factor corrector has a lower power factor and larger current harmonics. In addition, the buck power factor corrector is further compromised by more complex control means.

There is also a buck-boost converter circuit (as shown in FIG. 4) or a fly-back converter circuit (as shown in FIG. 5) for serving as a power factor corrector. The two types of converter circuits above although indeed achieve a better power factor, due to the fact that the current path in converter circuits does not allow energy from the power source to directly charge the direct-current link capacitor, they are both disadvantaged by having a larger storage requirement for the inductor, a larger volume and poorer efficiency caused by magnetic energy loss.

There is yet another power factor corrector formed by integrating a boost converter circuit and a buck converter circuit, as shown in FIG. 6. An active switch transistor Q1 performs buck conversion when an active switch transistor Q2 is off; the active switch transistor Q2 performs boost conversion when the active switch transistor Q1 is off. However, unless being implemented in a customized integrated for a specific use, such design is extreme complex and is rather highly unfeasible and unpractical.

All the abovementioned conventional power factor correctors are disfavored by one common disadvantage—a large energy-storing capacitor C_(DC) is required. To maintain a stable voltage at a load, an extremely small voltage change ΔV of the capacitor C_(DC) is needed, which means that it is necessary that the capacitor C_(DC) have sufficient capacitance for absorbing double-fold frequency power wave induced by the public electricity, as shown in FIG. 7. Limited by the extremely small voltage change ΔV for absorbing a difference between an input power and an output power, a capacitance of the direct-link capacitor needs to be very large.

TECHNICAL SUMMARY

A buck power factor correction system is provided according to an embodiment of the present invention. The buck power factor correction system comprises: a first storing device, for storing and discharging energy; a first converter device, coupled to the first storing device, for transferring and converter energy; a second storing device, coupled to the first storing device, for storing and discharging energy; and a second converter device, coupled to the second storing device, for transferring and converting energy. The buck power factor correction system further comprises: a rectifying device, coupled to the first storing device, receives and rectifies a power source to generate an input voltage; and a load, coupled to the first storing device. Preferably, the buck power factor correction system further comprises a switch element coupled between the first storing device and the second storing device.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1A is a boost converter circuit generally adopted in a conventional power factor corrector;

FIG. 1B is a schematic diagram of waveforms of an input voltage Vs and a current Is of the conventional power factor corrector in FIG. 1A;

FIG. 2 is a conventional power factor corrector comprising a two-order boost and buck converter circuits.

FIG. 3A is a conventional buck power factor corrector;

FIG. 3B is a schematic diagram of waveforms of an input voltage Vs and a current Is of the conventional power factor corrector in FIG. 3A;

FIG. 4 is a conventional power factor corrector comprising a buck-boost converter circuit;

FIG. 5 is a conventional power factor corrector comprising a fly-back converter circuit;

FIG. 6 is a power factor corrector integrating a boost and buck converter circuits;

FIG. 7 is a waveform diagram illustrating relations of power wave and a voltage change ΔV of an energy-storing capacitor C_(DC);

FIG. 8 is a buck power factor correction system according to an embodiment of the present invention;

FIGS. 9(A), 9(B) and 9(C) are a plurality of energy-transferring paths of the buck power factor correction system in FIG. 8;

FIG. 10 is an embodiment applying the structure shown in FIG. 8;

FIGS. 11(A), 11(B) and 11(C) illustrate circuit operations of the buck power factor correction system in FIG. 10 when an input voltage is greater than a voltage sum of first and second storing devices;

FIGS. 12(A), 12(B) and 12(C) illustrate circuit operations of the buck power factor correction system in FIG. 10 when an input voltage is smaller than a voltage sum of first and second storing devices but greater than a voltage of the first storing device;

FIGS. 13(A), 13(B) and 13(C) illustrate circuit operations of the buck power factor correction system in FIG. 10 when an input voltage is smaller than a voltage of the first storing device;

FIG. 14 is another embodiment applying the structure shown in FIG. 8;

FIGS. 15(A), 15(B) and 15(C) illustrate circuit operations of the buck power factor correction system in FIG. 14 when an input voltage is greater than a voltage sum of first and second storing devices;

FIGS. 16(A), 16(B) and 16(C) illustrate circuit operations of the buck power factor correction system in FIG. 14 when an input voltage is smaller than a voltage sum of first and second storing devices but greater than a voltage of the first storing device; and

FIGS. 17(A), 17(B) and 17(C) illustrate circuit operations of the buck power factor correction system in FIG. 14 when an input voltage is smaller than a voltage of the first storing device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of structures and concepts of the present invention shall be illustrated with embodiments below to give a better understanding on characteristics, objects and functions of the present invention.

Refer to FIG. 8 showing a buck power factor correction system 1 according to an embodiment of the present invention. The buck power factor correction system 1 comprises: a first storing device 11, a first converter device T₁, a second storing device 12, and a second converter device T₂. The first storing device 11 is for storing and discharging energy. The first converter device T₁ is coupled to the first storing device 11 is for transferring and converting energy. The second storing device 12 is coupled to the first storing device 11 is for storing and discharging energy. The second converter device T₂ is coupled to the second storing device 12 is for transferring and converting energy. The buck power factor correction system 1 further comprises a rectifying device 13 and a load. The rectifying device 13 is coupled to the first storing device 11 is for receives and rectifies a power source AC (e.g., an alternating current) to generate an input voltage V_(IN). The load is coupled to the first storing device 11.

Energy-storing paths of the first storing device 11 comprises: Path 1, for directly charging the first and second storing devices 11 and 12 by the input voltage V_(IN) to form a first energy-transferring path, as shown in FIG. 9(A); a Path 2, for charging the first storing device 11 via the first converter device T₁ by the input voltage V_(IN) to form a second energy-transferring path, as shown in FIG. 9(B); and a Path 3, for charging the first storing device 11 via the second converter device T₂ by the second storing device 12 to form a third energy-transferring path, as shown in FIG. 9(C).

The first converter device T₁ and the second converter device T₂ perform energy conversion according to a timing control, so that the energy-storing paths Path 1, Path 2 and Path 3 sequentially become effective. The energy needed by the load is supplied by the first storing device 11, and has a corresponding energy-transferring path of Path 4.

FIG. 10 shows an embodiment applying the structure shown in FIG. 8. A buck power factor correction system 2 comprises: a first storing device 21, a second storing device 22, a first converter device T₁ and a second active switch transistor Q₂. In practice, the first converter device T₁ may be combined with the second active switch transistor Q₂ to form a second converter device T₂. The buck power factor correction system 2 further comprises a rectifying device 23 and a load. The rectifying device 23 receives and rectifies a power source AC (e.g., an alternating current) to generate an input voltage V_(IN). In this embodiment, the first and second storing devices 21 and 22 may respectively be capacitors C₁ and C₂, the rectifying device 23 may be a bridge rectifier. The first converter device T₁ comprises a first diode D₁, a first active transistor Q₁ and a first inductor L₁; the second converter device T₂ comprises the first diode D₁, the first active switch transistor Q₁, the first inductor L1 and the second active switch transistor Q₂. Further, the first diode D₁ in the first converter device T₁ has its one end coupled to the rectifying device 23 and its other end coupled to the first active switch transistor Q₁ and the first inductor L₁. The first active switch transistor Q₁ has its one end coupled to the first diode D₁ and its other end coupled to the rectifying device 23. The first inductor L₁ has its one end coupled to the first diode D₁ and its other end coupled to the first storing device 21 (the capacitor C₁). The second active switch transistor Q₂ in the second converter device T₂ has its one end coupled to the first active switch transistor Q₁ and its other end coupled to the second storing device 22 (the capacitor C₂). The first storing device 21 (the capacitor C₁) has its one end coupled to the first diode D₁ and its other end coupled to the second storing device 22 (the capacitor C₂), and is connected in parallel with the load. The second storing device 22 (the capacitor C₂) has its one end coupled to the capacitor C₁ and its other end coupled to the second active switch transistor Q₂. The second active switch transistor Q₂ is responsible for low-frequency switching; the first active switch transistor Q₁ is controlled by a high-frequency pulse-width modulation (PWM) signal that modulates the pulse width through a feedback signal. Circuit operations of the buck power factor correction system 2 shall be described below.

A mode 1 shall be first discussed below. In this embodiment, when the input voltage V_(IN) is greater than a voltage sum V_(C1)+V_(C2) of the first and second storing devices 21 and 22, a bypass diode on the second active switch transistor Q₂ is forward biased, so that the input voltage V_(IN) directly charges the first and second storing devices 21 and 22 to form a first energy-transferring path Path 1. Meanwhile, the input voltage V_(IN) charges the first storing device 21 via the first transferring device T₁ to form a second energy-transferring path Path 2. A fourth path Path 4 is also formed between the first storing device 21 and the load, which has a cross voltage V_(O). Under this mode, the second converter device T₂ is non-operating, and so a third energy-transferring path Path 3 is non-existent. The first energy-transferring path Path 1 is formed from the input voltage V_(IN) via the first and second storing devices 21 and 22 (the capacitors C₁ and C₂) and the bypass diode connected to the second active switch transistor Q₂. The second energy-transferring path Path 2 is formed from the input voltage V_(IN) via the first storing device 21 (the capacitors C₁) and the first converter device T₁. The fourth energy-transferring path Path 4 is formed by a loop between the first storing device 21 (the capacitors C₁) and the load. Operation details of the circuit above are as depicted in FIG. 11(A).

Under the mode 1, when the first active switch transistor Q₁ in the first converter device T₁ is turned on by high-frequency PWM and the first diode D₁ is off, a second energy-transferring path Path 2 is formed. The second energy-transferring path Path 2 is formed by a loop of the input voltage V_(IN), the first storing device 21 (the capacitor C₁), the first inductor L₁ and the first active switch transistor Q₁, as shown in FIG. 11(A). The input voltage V_(IN) charges the first energy-storing capacitor C₁ and the first inductor L₁ at this point. When the high-frequency PWM at the first active switch transistor Q₁ is turned off, the first diode D₁ in the first converter device T₁ becomes turned on to form a new second energy-transferring path New Path 2, which is formed by a loop of the first energy-storing capacitor C₁, the first inductor L₁ and the first diode D₁. Further, the energy in the first inductor L₁ is discharged to the first energy-storing capacitor C₁ via the newly formed path New Path 2, as shown in FIG. 11(B). Meanwhile, the bypass diode on the second active switch transistor Q₂ remains turned on under this mode, so that the energy-transferring paths Path 1 and Path 4 stay unchanged.

Referring to FIG. 11(C), when the energy in the first inductor L₁ is completely discharged to the first energy-storing capacitor C₁ via the new second energy-transferring path New Path 2, the first diode D₁ and the first active switch transistor Q₁ become turned off such that the newly formed second energy-transferring path New Path 2 disappears, while the first and fourth energy-transferring paths Path 1 and 4 remain unchanged—such stage only takes place when the current of the first inductor L₁ is discontinuous. When the first active switch transistor Q₁ is again turned on by high-frequency PWM, the second energy-transferring path Path 2 is again formed to thus operate cyclically.

A mode 2 shall be discussed. In this embodiment, when the input voltage V_(IN) is smaller than a voltage sum V_(C1)+V_(C2) of the first and second storing devices 21 and 22, but greater than the voltage V_(C1) of the first energy-storing capacitor C₁, the second active switch transistor Q₂ is off, the first energy-transferring path Path 1 is non-existent and the second storing device 22 (the capacitor C₂) is no longer charged; the second converter device T₂ is non-operating, i.e., the third energy-transferring path Path 3 is non-existent. At this point, the input voltage V_(IN) charges the first storing device 21 (the capacitor C₁) via the first converter device T₁ to form a second energy-transferring path Path 2. Further, a fourth energy-transferring path Path 4 is formed between the first storing device 21 (the capacitor C₁) and the load, as shown in FIG. 12(A). Thus, the second energy-transferring path Path 2 is formed from the input voltage V_(IN) via the first storing device 21 (the capacitor C₁) and the first converter device T₁, and the fourth energy-transferring path Path 4 is formed by a loop between the first storing device 21 (the capacitor C₁) and the load.

Under the mode 2, when the first active switch transistor Q₁ in the first converter device T₁ is turned on by high-frequency PWM and the first diode D₁ is off, a second energy-transferring path Path 2 is formed. The second energy-transferring path Path 2 is formed by a loop of the input voltage V_(IN), the first storing device 21 (the capacitor C₁), the first inductor L₁ and the first active switch transistor Q₁. At this point, the input voltage V_(IN) charges the first energy-storing capacitor C₁ and the first inductor L₁. When the high-frequency PWM at the first active switch transistor Q₁ is turned off, the first diode D₁ in the first converter device T₁ becomes turned on to form a new second energy-transferring path New Path 2, which is formed by a loop of the first energy-storing capacitor C₁, the first inductor L₁ and the first diode D₁. Further, the energy in the first inductor L₁ is discharged to the first energy-storing capacitor C₁ via the newly formed path New Path 2, as shown in FIG. 12(B).

Referring to FIG. 12(C), when the energy in the first inductor L₁ is completely discharged to the first energy-storing capacitor C₁ via the new second energy-transferring path New Path 2, the first diode D₁ and the first active switch transistor Q₁ in the first converter device T₁ become turned off such that the newly formed second energy-transferring path New Path 2 disappears, while the fourth energy-transferring paths Path 4 remains unchanged—such stage only takes place when the current of the first inductor L₁ is discontinuous. When the first active switch transistor Q₁ is again turned on by high-frequency PWM, the second energy-transferring path Path 2 is again formed to thus operate cyclically.

Next, a mode 3 shall be discussed. In this embodiment, when the input voltage V_(IN) is smaller than the voltage V_(C1) of the first storing device 21 (the capacitor C₁), the second active switch transistor Q2 is turned on to activate the second converter device T₂, so as to form a third energy-transferring path Path 3. The third energy-transferring path Path 3 is formed by the second storing device 22 (the capacitor C₂) and the second converter device T₂. A fourth energy-transferring path Path 4 is also formed between the first storing device 21 (the capacitor C₁) and the load. Under this mode, the first energy-transferring path Path 1 and the second energy-transferring path Path 2 are non-existent. Further, the fourth energy-transferring path Path 4 is formed by a loop between the first storing device 21 (the capacitor C₁) and the load, as shown in FIG. 13(A).

Under the mode 3, when the first active switch transistor Q₁ in the second converter device T₂ is turned on by the high-frequency PWM and the first diode D₁ is off, a third energy-transferring path Path 3 is formed. The third energy-transferring path Path 3 is formed by a loop of the first inductor L₁, the first active switch transistor Q₁, the second active switch transistor Q₂ and the second storing device 22 (the capacitor C₂). At this point, the second energy-storing capacitor C₂ charges the first inductor L₁. Referring to FIG. 13(B), when the high-frequency PWM at the first active switch transistor Q₁ is turned off, the first diode D₁ in the second converter device T₂ becomes turned on to form a new third energy-transferring path New Path 3, which is formed by a loop of the first storing device 21 (the capacitor C₁), the first inductor L₁ and the first diode D₁. Further, the energy in the first inductor L₁ is discharged to the first energy-storing capacitor C₁ via the newly formed path New Path 3.

Referring to FIG. 13(C), when the energy in the first inductor L₁ is completely discharged to the first energy-storing capacitor C₁ via the new third energy-transferring path New Path 3, the first diode D₁ and the first active switch transistor Q₁ in the second converter device T₂ become turned off such that the newly formed third energy-transferring path New Path 3 disappears, while the fourth energy-transferring path Path 4 remains unchanged—such stage only takes place when the current of the first inductor L₁ is discontinuous. When the first active switch transistor Q₁ is again turned on by high-frequency PWM, the third energy-transferring path Path 3 is again formed to thus operate cyclically.

According to this embodiment, the first diode D₁, the first active switch transistor Q₁ and the first inductor L₁ compose the first converter device T₁. However, during circuit operations, the first diode D₁, the first active switch transistor Q₁, the first inductor L₁ and the second active switch transistor Q₂ may also compose the second converter device T₂.

FIG. 14 shows another embodiment applying the structure shown in FIG. 8. A buck power factor correction system 3 comprises a first storing device 31, a second storing device 32, a first converter device T₁ and a second converter device T₂. The buck power factor correction system 3 further comprises a rectifying device 33 and a load. The rectifying device 33 receives and rectifies a power source AC (e.g., an alternating current) to generate an input voltage V_(IN). The buck power factor correction system 3 further comprises a switch element 34 coupled between the first storing device 31 and the second storing device 32. In this embodiment, the first and second storing devices 31 and 32 may be capacitors C₁ and C₂; the rectifying device 33 may be a bridge rectifier; the first converter device T₁ comprises a first diode D₁, a first active switch transistor Q₁ and a first inductor L₁; the second converter device T₂ comprises a second diode D₂, a second active switch transistor Q₂, a second inductor L₂; and the switch element 34 may be a diode D₃. Further, the first diode D₁ in the first converter device T₁ has its one end coupled to the rectifying device 33 and its other end coupled to the first active switch transistor Q₁ and the first inductor L₁. The first active switch transistor Q₁ in the first converter device T₁ has its one end coupled to the first diode D₁ and its other end coupled to the rectifying device 33. The first inductor L₁ in the first converter device T₁ has its one end coupled to the first diode D₁ and its other end coupled to the switch element 34. The second diode D₂ in the second converter device T₂ has its one end coupled to the first diode D₁ and the first storing device 31 (the capacitor C₁), and its other end coupled to the Second active switch transistor Q₂ and the Second inductor L₂. The second active switch transistor Q₂ in the second converter device T₂ has its one end coupled to the second diode D₂, and its other end to the first active switch transistor Q₁ and the second storing device 32. The second inductor L₂ in the second converter device T₂ has its one end coupled to the second diode D₂ and its other end to the switch element 34. The first storing device 31 (the capacitor C₁) has its one end coupled to the first diode D₁ and its other end to the switch element 34, and is connected in parallel with the load. The capacitor C₂ in the second storing device 32 has its one end coupled to the switch element 34 and its other end coupled to the second active switch transistor Q₂. The first and second active switch transistors Q₁ and Q₂ may be both controlled by a set or respectively controlled by two sets of a high-frequency pulse-width modulation (PWM) signals that perform modulation through a feedback signal. Circuit operations of the buck power factor correction system 3 shall be described below.

A mode 1 shall first be discussed below. In this embodiment, when the input voltage V_(IN) is greater than a voltage sum V_(C1)+V_(C2) of the first and second storing devices 31 and 32, the switch element 34 (the diode D₃) is forward biased, so that the input voltage V_(IN) directly charges the first and second storing devices 31 and 32 to form a first energy-transferring path Path 1. Meanwhile, the input voltage V_(IN) charges the first storing device 21 via the first transferring device T₁ to form a second energy-transferring path Path 2. A fourth path Path 4 is also formed between the first storing device 31 and the load, which has a cross voltage V_(O). Under this mode, the second converter device T₂ is non-operating, and so a third energy-transferring path Path 3 is non-existent. Referring to FIG. 15(A), the first energy-transferring path Path 1 is formed from the input voltage V_(IN) via the first and second storing devices 31 and 32 (the capacitors C₁ and C₂) and the switch element 34 (the diode D₃). The second energy-transferring path Path 2 is formed from the input voltage V_(IN) via the first storing device 31 (the capacitors C₁) and the first converter device T₁. The fourth energy-transferring path Path 4 is formed by a loop between the first storing device 31 (the capacitors C₁) and the load.

Under the mode 1, when the first active switch transistor Q₁ in the first converter device T₁ is turned on by high-frequency PWM and the first diode D₁ is off, a second energy-transferring path Path 2 is formed. The second energy-transferring path Path 2 is formed by a loop of the input voltage V_(IN), the first storing device 31 (the capacitor C₁), the first inductor L₁ and the first active switch transistor Q₁. Referring to FIG. 15(A), the input voltage V_(IN) charges the first energy-storing capacitor C₁ and the first inductor L₁ at this point. When the high-frequency PWM at the first active switch transistor Q₁ is turned off, the first diode D₁ in the first converter device T₁ becomes turned on to form a new second energy-transferring path New Path 2, which is formed by a loop of the first energy-storing capacitor device 31 (the capacitor C₁), the first inductor L₁ and the first diode D₁, as shown in FIG. 15(B). The energy in the first inductor L₁ is discharged to the first energy-storing capacitor C₁ via the newly formed path New Path 2. Meanwhile, the energy-transferring paths Path 1 and Path 4 stay unchanged.

Referring to FIG. 15(C), when the energy in the first inductor L₁ is completely discharged to the first energy-storing capacitor C₁ via the new second energy-transferring path New Path 2, the first diode D₁ and the first active switch transistor Q₁ in the first converter device T₁ become turned off such that the newly formed second energy-transferring path New Path 2 disappears, while the first and fourth energy-transferring paths Path 1 and 4 remain unchanged—such stage only takes place when the current of the first inductor L₁ is discontinuous. When the first active switch transistor Q₁ is again turned on by high-frequency PWM, the second energy-transferring path Path 2 is again formed to thus operate cyclically.

A mode 2 shall be discussed. In this embodiment, when the input voltage V_(IN) is smaller than a voltage sum V_(C1)+V_(C2) of the first and second storing devices 31 and 32 but greater than the voltage V_(C1) of the first energy-storing capacitor C₁, the switch element 34 (the diode D₃) is reverse biased, the first energy-transferring path Path 1 is non-existent and the second storing device 32 (the capacitor C₂) is no longer charged; the second converter device T₂ is non-operating, i.e., the third energy-transferring path Path 3 is non-existent. At this point, the input voltage V_(IN) charges the first storing device 31 (the capacitor C₁) via the first converter device T₁ to form a second energy-transferring path Path 2. Further, a fourth energy-transferring path Path 4 is formed between the first storing device 31 (the capacitor C₁) and the load, as shown in FIG. 16(A). The second energy-transferring path Path 2 is formed from the input voltage V_(IN) via the first storing device 31 (the capacitor C₁) and the first converter device T₁, and the fourth energy-transferring path Path 4 is formed by a loop between the first storing device 31 (the capacitor C₁) and the load.

Under the mode 2, when the first active switch transistor Q₁ in the first converter device T₁ is turned on by high-frequency PWM and the first diode D₁ is off, a second energy-transferring path Path 2 is formed. The second energy-transferring path Path 2 is formed by a loop of the input voltage V_(IN), the first storing device 31 (the capacitor C₁), the first inductor L₁ and the first active switch transistor Q₁. At this point, the input voltage V_(IN) charges the first energy-storing capacitor C₁ and the first inductor L₁. When the high-frequency PWM at the first active switch transistor Q₁ is turned off, the first diode D₁ in the first converter device T₁ becomes turned on to form a new second energy-transferring path New Path 2, which is formed by a loop of the first energy-storing capacitor C₁, the first inductor L₁ and the first diode D₁. Further, the energy in the first inductor L₁ is discharged to the first energy-storing capacitor C₁ via the newly formed path New Path 2, as shown in FIG. 16(B).

Referring to FIG. 16(C), when the energy in the first inductor L₁ is completely discharged to the first energy-storing capacitor C₁ via the new second energy-transferring path New Path 2, the first diode D₁ and the first active switch transistor Q₁ become turned off such that the newly formed second energy-transferring path New Path 2 disappears, while the fourth energy-transferring paths Path 4 remains unchanged—such stage only takes place when the current of the first inductor L₁ is discontinuous. When the first active switch transistor Q₁ is again turned on by high-frequency PWM, the second energy-transferring path Path 2 is again formed to thus operate cyclically.

Next, a mode 3 shall be discussed. In this embodiment, when the input voltage V_(IN) is smaller than the voltage V_(C1) of the first storing device 31 (the capacitor C₁), the second converter device T2 starts to operate to form a third energy-transferring path Path 3. The third energy-transferring path Path 3 is formed by the second storing device 32 (the capacitor C₂) and the second converter device T₂. A fourth energy-transferring path Path 4 is also formed between the first storing device 31 (the capacitor C₁) and the load. Under this mode, the first energy-transferring path Path 1 and the second energy-transferring path Path 2 are non-existent. Further, the fourth energy-transferring path Path 4 is formed by a loop between the first storing device 31 (the capacitor C₁) and the load, as shown in FIG. 17(A).

Under the mode 3, when the second active switch transistor Q₂ in the second converter device T₂ is turned on by high-frequency PWM and the second diode D₂ is off, a third energy-transferring path Path 3 is formed. The third energy-transferring path Path 3 is formed by a loop of the second inductor L₂, the second active switch transistor Q₂ and the second storing device 32 (the capacitor C₂). The second energy-storing capacitor C₂ charges the second inductor L₂ at this point. When the high-frequency PWM at the second active switch transistor Q₂ is turned off, the second diode D₂ in the second converter device T₂ becomes turned on to form a new third energy-transferring path New Path 3, which is formed by a loop of the first storing device 31 (the capacitor C₁), the second inductor L₂, the second diode D₂ and the switch element 34 (the diode D₃). Further, the energy in the second inductor L₂ is discharged to the first energy-storing capacitor C₁ via the newly formed path New Path 3, as shown in FIG. 17(B).

Referring to FIG. 17(C), when the energy in the second inductor L₂ is completely discharged to the first storing device 31 (the capacitor C₁) via the new third energy-transferring path New Path 3, the second diode D₂ and the second active switch transistor Q₂ in the second converter device T₂ become turned off such that the newly formed second energy-transferring path New Path 3 disappears, while the fourth energy-transferring path Path 4 remains unchanged—such stage only takes place when the current of the second inductor L₂ is discontinuous. When the second active switch transistor Q₂ is again turned on by high-frequency PWM, the third energy-transferring path Path 3 is again formed to thus operate cyclically.

With description of the above embodiments, it is illustrated that the present invention is capable of directly charging a direct-link capacitor by partial energy via a current path, so as to decrease an energy-storing requirement of an inductor, reduce a volume of an inductor and optimize circuit efficiency, thereby simplifying an overall circuit structure that is then easier to control. Further, in the present invention, the energy-storing capacitance is divided into two capacitors—a first capacitor, connected in parallel with a load to stabilize the load voltage, and a second capacitor, connected in series with the first capacitor to form a voltage divider for reducing a voltage stress and a volume of the first energy-storing capacitor. The second energy-storing capacitor transfers energy to the first energy-storing capacitor via a converter circuit, so as to assist the first energy-storing capacitor in stabilizing the voltage and thus to reduce the volume of the first energy-storing capacitor. Further, since the second energy-storing capacitor allows a voltage change ΔV of a large range or even complete energy discharge, the required capacitance of the second energy-storing capacitor may also be decreased.

While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the above embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. 

What is claimed is:
 1. A buck power factor correction system, comprising: a first storing device, for storing and discharging energy; a first converter device, coupled to the first storing device, for transferring and converting energy; a second storing device, coupled to the first storing device, for storing and discharging energy; and a second converter device, coupled to the second storing device, for transferring and discharging energy.
 2. The buck power factor correction system according to claim 1, further comprising: a rectifying device, coupled to the first storing device, for receiving and rectifying a power source to generate an input voltage; and a load, coupled to the first storing device.
 3. The buck power factor correction system according to claim 2, wherein the first storing device comprises a plurality of energy-storing paths as a plurality of energy-transferring paths.
 4. The buck power factor correction system according to claim 3, wherein the plurality of energy-transferring paths comprise: a first energy-transferring path, formed by directly charging the first and second storing devices by the input voltage; a second energy-transferring path, formed by charging the first storing device via the first converter device by the input voltage; and a third energy-transferring path, formed by charging the first storing device via the second converter device by the second storing device.
 5. The buck power factor correction system according to claim 1, wherein the first and second storing devices are capacitors.
 6. The buck power factor correction system according to claim 2, wherein the rectifying device is a bridge rectifier.
 7. The buck power factor correction system according to claim 1, wherein the first converter device comprises a first diode, a first active switch transistor and a first inductor; the second converter device comprises the first diode, the first active transistor, the first inductor and a second active switch transistor.
 8. The buck power factor correction system according to claim 7, wherein the first energy-transferring path is formed from the input voltage via the first and second storing devices and a bypass diode on the second active switch transistor; the second energy-transferring path is formed from the input voltage via the first storing device and the first converter device; and the third energy-transferring path is formed from the second storing devices via the second converter device and the first storing device.
 9. The buck power factor correction system according to claim 8, wherein between the first storing device and the load forms a fourth energy-transferring path.
 10. The buck power factor correction system according to claim 7, wherein the second active switch transistor is for low-frequency switching, the first active switch transistor is controlled by high-frequency pulse width modulation (PWM), and the pulse width is modulated via a feedback signal.
 11. The buck power factor correction system according to claim 3, wherein the first converter device comprises a first diode, a first active switch transistor and a first inductor; the second converter device comprises a second diode, a second active switch transistor and a second inductor.
 12. The buck power factor correction system according to claim 11, further comprising: a switch element, coupled between the first storing device and the second storing device.
 13. The buck power factor correction system according to claim 12, wherein the switch element is a semiconductor diode.
 14. The buck power factor correction system according to claim 13, wherein the first energy-transferring path is formed from the input voltage via the first and second storing devices and the semiconductor diode; the second energy-transferring path is formed from the input voltage via the first storing device and the first converter device; and the third energy-transferring device is formed by the first storing device, the second storing device and the second converter device.
 15. The buck power factor correction system according to claim 14, wherein between the first storing device and the load forms a fourth energy-transferring path.
 16. The buck power factor correction system according to claim 11, wherein the first active switch transistor and the second active switch transistor are controlled by high-frequency PWM, and the pulse width is modulated via a feedback signal.
 17. The buck power factor correction system according to claim 1, wherein the energy is a voltage or a current.
 18. The buck power factor correction system according to claim 7, wherein the first diode in the first converter device is coupled to the rectifying device, the first active switch transistor in the first converter device has one end coupled to the first diode and one other end coupled to the rectifying device, the first inductor in the first converter device has one end coupled to the first diode and one other end coupled to the first storing device.
 19. The buck power factor correction system according to claim 18, wherein the second active switch transistor in the second converter device has one end coupled to the first active switch transistor and one other end coupled to the second storing device.
 20. The buck power factor correction system according to claim 12, wherein the first diode in the first converter device is coupled to the rectifying device, the first active switch transistor in the first converter device has one end coupled to the first diode and one other end coupled to the rectifying device, and the first inductor in the first converter device has one end coupled to the first diode and one other end coupled to the switch element.
 21. The buck power factor correction system according to claim 20, wherein the second diode in the second converter device is coupled to the first diode, the second active switch transistor in the second converter device has one end coupled to the second diode and one other end coupled to the first active switch transistor, and the second inductor in the second converter device has one end coupled to the second diode and one other end coupled to the switch element. 